Tumors harbor dynamic microenvironments in which cancer cells are intimately associated with non-transformed host cells. The tumor-associated stroma is considered to play an important role during tumor growth, influencing phenomena such as angiogenesis, metastasis and immune suppression.(36) As such, the stroma forms an attractive target for diagnostic and therapeutic applications.(37) 
Different myeloid cells are important components of the tumor stroma. Myeloid cells are frequently found to infiltrate tumors and have been linked to diverse tumor-promoting activities.(1) In particular, tumor-associated macrophages (TAMs) are an important component of the tumor stroma, both in murine models and human patients.(2) TAMs can promote tumor-growth by affecting angiogenesis, immune suppression and invasion and metastasis.(2,3) 
Tissue-resident macrophages can be maintained through local proliferation or differentiation in situ from circulating monocytic precursors.(5) Importantly, discrete subsets of blood monocytes have been described. Mouse monocytes can be classified as Ly6ClowCX3CR1hi(CCR2−CD62L−) or Ly6ChiCX3CR1low(CCR2+CD62L+) and are shown to have distinct functions and migration patterns.(6) 
Macrophages are plastic cells that can adopt different phenotypes depending on the immune context. Microenvironmental stimuli can drive a macrophage either towards a “classical” (M1) or an “alternative” (M2) activation state, two extremes in a spectrum.(7) M1 macrophages are typically characterized by the expression of pro-inflammatory cytokines, inducible nitric oxide synthase 2 (Nos2) and MHC Class II molecules. M2 macrophages have a decreased level of the aforementioned molecules and are identified by their signature-expression of a variety of markers, including arginase-1 and mannose and scavenger receptors. It has been suggested that TAMs display a M2-like phenotype.(8) 
Despite the presence of TAM in tumor infiltrate and their potential to produce angiogenic factors, their role in tumor growth and development remains unclear. There remains a need to discover and understand the complexities of the tumor-infiltrating myeloid cell compartment in view of the selective treatment of tumor growth.